The present invention relates to a device for detecting photographic image information on an original film in a photographic printing system and so on, and more particularly to a photographic image information detecting device which is capable of obtaining an apparently wide dynamic range even if a storage-type light receiving element having a narrow dynamic range is used (e.g., a two-dimensional area image sensor or a one-dimensional line sensor comprising a charge transfer device such as CCD).
In a photographic printer, it is necessary to measure the density of an original film (negative or positive film) in order to determine the exposure or correction required for printing the film. In the prior art, the average density of a negative film is photographically measured in LATD (Large Area Transmittance Density) by a photo-sensor such as a photodiode provided near the light pass of an optical system. This image detection in LATD uniformly determines the photometric value of the negative films, but does not precisely measure the density of images over all of the frame. This leads to such drawbacks as incorrect exposure or uncertain correction. There has also been proposed a method of segmenting a frame of a negative film horizontally and vertically into several segments, photographically measuring each of them to determine their necessary exposure, and finally correcting the exposure. However, since a photometric determination by this method is not quite precise, it cannot detect accurate image information about details of the images.
The image information detecting device for determining the density of an image must cover a wide dynamic range corresponding to the density range of an original film. It should also cover differences in lamps and filters used in the photometric system. The necessary dynamic range therefor amounts to about 1:10,000. It is also important for a printer to detect information from a relatively wide area of a frame for determining exposure or correction amount.
Such requirements are conventionally satisfied mainly with scanner-type density information detecting devices which are used as a light receiving element of a relatively wide dynamic range such as a photodiode and photomultiplier for scanning negative films. But a detecting device provided with such a scanning mechanism is complex in structure leading to a high cost in installation and a high rate of failures, and also requires much time for processing. The low processing speed in the whole system is attributable mainly to the large amount of time required for image processing, thus presenting a significant problem in the system.
The light receiving elements may comprise a charge transfer device (hereinafter referred to as a storage-type light receiving element) such as CCD (Charge Coupled Device) or BBD (Bucket Brigade Device) for electrical scanning, but there still remain a problem because the light receiving element of the storage-type has a particularly narrow dynamic range. If such an element is used, a dynamic range which is wide enough to carry out detection of the density information of negative films cannot be obtained.